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AdrianTheRock
#3
Mar12-12, 05:36 PM
P: 136
Quote Quote by WilliamJ View Post
...The first question is about a red quark absorbing an red-antiblue quark and turning blue. I don't see how a gluon of red-anti-blue quark makes the red quark suddenly turn blue, not anti-blue...
You're right, this particular scenario cannot occur. A red quark could emit a red-antiblue gluon to become a blue quark - sort of like "+red - (+red + -blue) = +blue" - but for a red quark to become blue by absorbing a gluon the latter must be blue-antired.
...but I have heard that the quarks can only have a regular color and an anti-quark are the only quarks that can have an anti-color. "As a quark absorbs a gluon, it keeps its status as a quark or anti-quark. So what does that mean?
It means that, apart from its colour, all of the other charges and internal quantum numbers of the quark remain the same. Eg if its flavour is up it remains up. And, in particular, gluons cannot change quarks into antiquarks, just as photons cannot change (eg) electrons into positrons.
Also, if a blue quark ineracts with another quark of a different color say red, the qluon is transfered and the quarks swap their colors. This means that when two quarks exchange a gluon, they switch their colors, but what about when they are exchanging gluons that are like the red and and antiblue quarks?
This is one and the same thing - quarks change colours by exchanging gluons. The word "exchanging" as used here is not intended to imply that each interaction involves sending gluons in both direction. Only one gluon need be transferred in each interaction.

Gluons can be also exchanged between a quark and an anti-quark. For example a green quark can emit a green-antiblue gluon (thus becoming blue) and the gluon can then be absorbed by an antigreen antiquark, turning the latter antiblue. This is a typical interaction within a meson - notice how the anti-quark both starts and ends having the anti- of the quark's colour.
...And the question lingers, what about quarks and anti-quarks and gluons that are colors and ant-colors and how they relate?
The simple picture of red-antiblue type gluons unfortunately isn't the whole story.

In fact, in addition to the six gluons of the colour - anti-other-colour combinations, there are two further ones which can loosely be described as "neutral". Their colour combinations can be described mathematically as
[itex]1\ /\ \sqrt{2}\ (r\bar{r}\ -\ g\bar{g})\ \ \ \ [/itex]and[itex]\ \ \ \ 1\ /\ \sqrt{6}\ (r\bar{r}\ +\ g\bar{g}\ -\ 2b\bar{b})[/itex]
These can be exchanged between any quarks or anti-quarks that have at least one of their (anti-)colours.

Even the above, however, is still a very oversimplified picture. To really understand this you really do have to have at least a basic understanding of the SU(3) symmetry group. There is quite a good introduction to gluon types and exchanges in "Elementary Particle Physics" by Alessandro Bettini.